1. Field of the Invention
The subject invention relates to a variable displacement compressors and more specifically to an improved pivot assembly having a hinge for these types of compressors.
2. Description of the Prior Art
Variable displacement compressors, such as those shown in U.S. Pat. Nos. 6,139,283 and 6,402,481, are well known in the art. These types of compressors typically include a rotor secured to a drive shaft. A swash plate is slidably mounted on the drive shaft and is operatively engaged with the rotor. The engagement mechanism between the swash plate and rotor consists of one or more cylindrical pin(s) rigidly fixed to the swash plate that slidingly engage corresponding slots machined on the rotor. Alternately, other prior art designs utilize two inclined cylindrical bores that are machined features on the rotor which slidingly engage with steel (spherical) balls that extend on support posts from the swash plate. In either case, both interfaces support high forces and generate very high contact stresses; consequently the cast iron surfaces of the slots (or bores) must be heat treated for wear resistance against the hard steel pins (or sphere & post), which adds significant manufacturing cost. Furthermore, the high contact stress produces friction that resists the stroke movement of the mechanism, causing erratic controllability of compressor displacement.
In addition to the above, a hinge, having a pair of pins, may be used to pivotally engage between the swash plate and the rotor. The hinge has the advantage over the slot and pin arrangement (or bore with ball) because the sliding interface is replaced with two rotary pin connections which significantly reduces the contact stresses, allowing the joints to move freely with less frictional resistance under the same imposed loads. The hinge mechanism thus improves the controllability of the compressor displacement.
During operation of the variable displacement compressors, the hinge & pin assembly must support the compressive force generated by the pistons, and also withstand the lateral forces that react at opposite ends of the hinge, which are generated by the shaft torque pulses. These dynamic lateral side forces cause chatter across the assembly clearances on the side of the hinge interface. This vibration movement will attempt to displace the pin along its axis from the hinge; therefore, the pins must be rigidly secured to the hinge. This can be accomplished by interference fit of the pins to the mating ends of the hinge; other means include the addition of retaining rings attached to the end of the pins, to prevent movement relative the final assembly, but this method will have added cost, assembly complexity, and increased package size. Another requirement of the hinge and pin assembly is that it must low weight; the weight of the hinge and pins must be counterbalanced by added material to the swash plate and rotor, in order to minimize centrifugal forces and maintain mechanism inertial balance, which is required for high-speed stroke control stability. The prior art hinges are thus made from aluminum; an iron or steel hinge of this same geometry would be impractical to balance given the limited space and the weight restrictions imposed on the modern compact variable compressor. Typically, the pins are made from hardened steel for wear resistance against the cast iron holes on the rotor and swash plate. In addition, the aluminum hinge must possess high tensile strength to withstand the stresses caused by the interference fit pin, and the compression loads that react across the pins. The interference fit must be large enough to retain the pins at high operating temperature since the aluminum expands at twice the rate as the steel pin, which would otherwise allow the pin to vibrate loose. However, if the interference is excessive, the hinge may fracture due to the thermal contraction when the compressor is exposed to very low ambient conditions. Another consideration regarding the aluminum alloy is that it must have high silicon content for wear resistance at the lateral surfaces that engage with the mating cast iron ear surfaces of the rotor and swash plate. A final consideration is that the leading ends of the pins must be well rounded (added cost for machining/tumbling) to prevent galling/plowing of the aluminum during the press-fit assembly, which is aggravated by the high interference fit.
In summary, the prior art hinge must have precision-machined holes with very narrow tolerances, and it must be made of high strength extruded aluminum bar to withstand the press fit and remain lightweight. Also, the lateral surfaces normal to the holes must be precision machined to minimize wear at the mating cast iron rotor and swash plate ears, which also increases costs.
Accordingly, it would be desirable to provide a hinge which is relatively lightweight, inexpensive, easy to manufacture, and can withstand the stresses imparted by the compressor.